Laser mirrors must be free from distortion of the reflective surface. It has been the practice to make the mirror substrate of a material having high heat conductivity, a high modulus of elasticity and a low coefficient of expansion, such as molybdenum. A coolant is circulated close to the mirror surface to prevent surface temperature build-up and to equalize the temperature throughout the substrate. The size and spacing of the coolant passages is critical in reducing both thermal and pressure distortion of the mirror surface. This requires a network of very small, closely spaced fluid passages close to the surface of the mirror. Making such a substrate by machining coolant passages in a single block of metal or forming the substrate from several plates of metal which are machined and brazed together is an extremely costly method of producing high-performance laser mirrors.